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1Lasers: Fundamentals, Types, and OperationsSubhash Chandra Singh, Haibo Zeng, Chunlei Guo, and Weiping Cai

The acronym LASER, constructed from Light Amplification by Stimulated Emissionof Radiation, has become so common and popular in every day life that it is nowreferred to as laser. Fundamental theories of lasers, their historical developmentfrom milliwatts to petawatts in terms of power, operation principles, beam char-acteristics, and applications of laser have been the subject of several books [15].Introduction of lasers, types of laser systems and their operating principles, meth-ods of generating extreme ultraviolet/vacuum ultraviolet (EUV/VUV) laser lights,properties of laser radiation, and modification in basic structure of lasers are themain sections of this chapter.

1.1Introduction of Lasers

1.1.1Historical Development

The first theoretical foundation of LASER and MASER was given by Einsteinin 1917 using Planks law of radiation that was based on probability coefficients(Einstein coefficients) for absorption and spontaneous and stimulated emissionof electromagnetic radiation. Theodore Maiman was the first to demonstrate theearliest practical laser in 1960 after the reports by several scientists, including thefirst theoretical description of R.W. Ladenburg on stimulated emission and negativeabsorption in 1928 and its experimental demonstration by W.C. Lamb and R.C.Rutherford in 1947 and the proposal of Alfred Kastler on optical pumping in 1950and its demonstration by Brossel, Kastler, and Winter two years later. Maimans firstlaser was based on optical pumping of synthetic ruby crystal using a flash lampthat generated pulsed red laser radiation at 694 nm. Iranian scientists Javan andBennett made the first gas laser using a mixture of He and Ne gases in the ratio of1 : 10 in the 1960. R. N. Hall demonstrated the first diode laser made of galliumarsenide (GaAs) in 1962, which emitted radiation at 850 nm, and later in the sameyear Nick Holonyak developed the first semiconductor visible-light-emitting laser.

Nanomaterials: Processing and Characterization with Lasers, First Edition.Edited by Subhash Chandra Singh, Haibo Zeng, Chunlei Guo, and Weiping Cai. 2012 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2012 by Wiley-VCH Verlag GmbH & Co. KGaA.

2 1 Lasers: Fundamentals, Types, and Operations

1.1.2Basic Construction and Principle of Lasing

Basically, every laser system essentially has an active/gain medium, placed betweena pair of optically parallel and highly reflecting mirrors with one of them partiallytransmitting, and an energy source to pump active medium. The gain media may besolid, liquid, or gas and have the property to amplify the amplitude of the light wavepassing through it by stimulated emission, while pumping may be electrical oroptical. The gain medium used to place between pair of mirrors in such a way thatlight oscillating between mirrors passes every time through the gain medium andafter attaining considerable amplification emits through the transmitting mirror.

Let us consider an active medium of atoms having only two energy levels: excitedlevel E2 and ground level E1. If atoms in the ground state, E1, are excited to theupper state, E2, by means of any pumping mechanism (optical, electrical discharge,passing current, or electron bombardment), then just after few nanoseconds oftheir excitation, atoms return to the ground state emitting photons of energyh = E2 E1. According to Einsteins 1917 theory, emission process may occur intwo different ways, either it may induced by photon or it may occur spontaneously.The former case is termed as stimulated emission, while the latter is knownas spontaneous emission. Photons emitted by stimulated emission have the samefrequency, phase, and state of polarization as the stimulating photon; therefore theyadd to the wave of stimulating photon on a constructive basis, thereby increasingits amplitude to make lasing. At thermal equilibrium, the probability of stimulatedemission is much lower than that of spontaneous emission (1 : 1033), thereforemost of the conventional light sources are incoherent, and only lasing is possiblein the conditions other than the thermal equilibrium.

1.1.3Einstein Relations and Gain Coefficient

Consider an assembly of N1 and N2 atoms per unit volume with energies E1and E2(E2 > E1) is irradiated with photons of density = N h, where [N] is thenumber of photons of frequency per unit volume. Then the stimulated absorptionand stimulated emission rates may be written as N1vB12 and N2vB21 respectively,where B12 and B21 are constants for up and downward transitions, respectively,between a given pair of energy levels. Rate of spontaneous transition depends onthe average lifetime, 21, of atoms in the excited state and is given by N2A21, whereA21 is a constant. Constants B12, B21, and A21 are known as Einstein coefficients.Employing the condition of thermal equilibrium in the ensemble, Boltzmannstatistics of atomic distribution, and Plancks law of blackbody radiation, it is easyto find out B12 = B21, A21 = B21(8h3/c3), known as Einstein relations, and ratio,R = exp(h/kT) 1, of spontaneous and stimulated emissions rates. For example,if we have to generate light of 632.8 nm ( = 4.74 1014 Hz) wavelength at roomtemperature from the system of HeNe, the ratio of spontaneous and stimulatedemission will be almost 5 1026, which shows that for getting strong lasing one

1.1 Introduction of Lasers 3

has to think apart from the thermal equilibrium. For shorter wavelength, laser,ratio of spontaneous to stimulated emission is larger, ensuring that it is moredifficult to produce UV light using the principle of stimulated emission comparedto the IR. Producing intense laser beam or amplification of light through stimulatedemission requires higher rate of stimulated emission than spontaneous emissionand self-absorption, which is only possible for N2 > N1 (as B12 = B21) even thoughE2 > E1 (opposite to the Boltzmann statistics). It means that one will have to createthe condition of population inversion by going beyond the thermal equilibrium toincrease the process of stimulated emission for getting intense laser light.

If a collimated beam of monochromatic light having initial intensity I0 passesthrough the mentioned active medium, after traveling length x, intensity ofthe beam is given by I(x) = I0ex, where is the absorption coefficient of themedium, which is proportional to the difference of N1 and N2. In the case ofthermal equilibrium N1 N2 the irradiance of the beam will decrease with thelength of propagation through the medium. However, in the case of populationinversion, (N2 > N1) , will be positive and the irradiance of the beam willincrease exponentially as I(x) = I0ekx, where k is the gain coefficient of themedium and may be given by k = (nNdh21B21)/c, where Nd is N2N1, c is speedof light, and n is refractive index of the medium.

1.1.4Multilevel Systems for Attaining Condition of Population Inversion

Considering the case of two energy level system under optical pumping, wehave already discussed that B12 = B21, which means that even with very strongpumping, population distribution in upper and lower levels can only be made equal.Therefore, optical as well as any other pumping method needs either three or fourlevel systems to attain population inversion. A three level system (Figure 1.1a)irradiated by intense light of frequency 02 causes pumping of large number ofatoms from lowest energy level E0 to the upper energy level E2. Nonradiative decayof atoms from E2 to E1 establishes population inversion between E1 and E0 (i.e.,N1 > N0), which is practically possible if and only if atoms stay for longer time inthe state E1 (metastable state, i.e., have a long lifetime) and the transition from E2 toE1 is rapid. If these conditions are satisfied, population inversion will be achievedbetween E0 and E1, which makes amplification of photons of energy E1 E0 bystimulated emission. Larger width of the E2 energy level could make possibleabsorption of a wider range of wavelengths to make pumping more effective, whichcauses increase in the rate of stimulated emission. The three level system needsvery high pumping power because lower level involved in the lasing is the groundstate of atom; therefore more than half of the total number of atoms have to bepumped to the state E1 before achieving population inversion and in each of thecycle, energy used to do this is wasted. The pumping power can be greatly reducedif the lower level involved in the lasing is not ground state, which requires at least afour level system (Figure 1.1b). Pumping transfers atoms from ground state to E3,from where they decay rapidly into the metastable state E2 to make N2 larger than

4 1 Lasers: Fundamentals, Types, and Operations

NN

E0

EE1

E3

E0

E

E1

E2Fast decay

Fas

t dec

ay

Fast decay

Pum

ping

Pum

ping

Lasin

g

Lasin

g

(a) (b)

Figure 1.1 Energy level diagram for (a) three- and (b) four level laser systems.

N1 to achieve the condition of population inversion between E2 and E1 at moderatepumping.

1.1.5Threshold Gain Coefficient for Lasing

Laser beam undergoes multiple oscillations (through active medium) between pairof mirrors to achieve considerable gain before it leaves the cavity through partiallyreflecting mirror. Laser oscillation can only sustain in the active medium if it attainsat least unit gain after a round-trip between mirrors and maintains it overcomingvarious losses inside the cavity. If we incorporate these losses, the effective gaincoefficient reduces to k , where is the loss coefficient of the medium. Ifround-trip gain G were less than unity, the laser osc